Package on Package (PoP) is an integrated circuit packaging technique that allows two or more integrated circuit packages, such as separate logic and memory packages, to be stacked on top of one another. Between the packages is an interface for transmitting signals between them. The interface typically includes a ball grid array (BGA) on the bottom of the top package. The ball grid array includes solder balls attached to the bottom of the top package. The interface also includes conductor pads such copper pads on the top surface of the bottom package. The conductor pads are arranged in a pattern that matches the solder balls. When the packages are assembled, the package assembly is heated, either in a reflow oven or by an infrared heater, causing the solder balls to melt and adhere to the conductor pads to connect the top and bottom packages both electrically and physically.
A similar connection can be made between the logic package and a motherboard with the BGA on the bottom of the logic package and conductor pads on the top of the motherboard.
A main benefit of PoP is to save space on the motherboard. Another is to decouple the memory device from the logic device, allowing different combinations of memory and logic packages. This means memory packages from different suppliers can be used with different logic packages. Furthermore, the memory and logic packages can be tested separately or together.
When a PoP assembly is tested, the packages are not soldered together so that the assembly can be disassembled and the packages can be used again with other packages. Instead, an interconnect is placed between the packages to temporarily connect the solder balls of one package to the conductor pads of the other package. An interconnect typically includes vertically placed probe pins that extends from the top surface of the interconnect to the bottom surface. When the packages are assembled with the interconnect disposed therebetween, one end of each pin is connected to a solder ball on the bottom of the top package, and the other end of the probe pin is connected to the corresponding pad on the top of the bottom package, to provide an electrical connection between the solder ball and the pad.
When probe pins are used to connect the two packages, the probe pins are pressed into the solder balls and pads of the packages, making it difficult for the packages to have relative movement. Additionally the solder balls and pads of the packages may be damaged by the probe pins during repeated use.